Method of processing polycrystalline diamond material

ABSTRACT

A method of processing a polycrystalline diamond (PCD) material having a non-diamond phase comprising a diamond catalyst/solvent material comprises leaching an amount of the diamond catalyst/solvent from the PCD material by exposing at least a portion of the PCD material to a leaching mixture. The leaching mixture comprises hydrochloric acid and one or more additional mineral acids, the molar concentration of the hydrochloric acid being greater than the molar concentration of the one or more additional mineral acids in the leaching mixture.

FIELD

This disclosure relates to a method of processing a body of polycrystalline diamond (PCD) material and to a mixture for said processing.

BACKGROUND

Cutter inserts for machining and other tools may comprise a layer of polycrystalline diamond (PCD) bonded to a cemented carbide substrate. PCD is an example of a superhard material, also called superabrasive material, which has a hardness value substantially greater than that of cemented tungsten carbide.

Components comprising PCD are used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. PCD comprises a mass of substantially inter-grown diamond grains forming a skeletal mass, which defines interstices between the diamond grains. PCD material comprises at least about 80 volume % of diamond and may be made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, typically about 5.5 GPa, and temperature of at least about 1200° C., typically about 1440° C., in the presence of a sintering aid, also referred to as a catalyst material for diamond. Catalyst material for diamond is understood to be material that is capable of promoting direct inter-growth of diamond grains at a pressure and temperature condition at which diamond is thermodynamically more stable than graphite.

Examples of catalyst materials for diamond are cobalt, iron, nickel and certain alloys including alloys of any of these elements. PCD may be formed on a cobalt-cemented tungsten carbide substrate, which may provide a source of cobalt catalyst material for the PCD. During sintering of the body of PCD material, a constituent of the cemented-carbide substrate, such as cobalt from a cobalt-cemented tungsten carbide substrate, liquefies and sweeps from a region adjacent the volume of diamond particles into interstitial regions between the diamond particles. In this example, the cobalt acts as a catalyst to facilitate the formation of bonded diamond grains. Optionally, a metal-solvent catalyst may be mixed with diamond particles prior to subjecting the diamond particles and substrate to the HPHT process. The interstices within PCD material may at least partly be filled with the catalyst material. The intergrown diamond structure therefore comprises original diamond grains as well as a newly precipitated or re-grown diamond phase, which bridges the original grains. In the final sintered structure, catalyst/solvent material generally remains present within at least some of the interstices that exist between the sintered diamond grains.

The sintered PCD has sufficient wear resistance and hardness for use in aggressive wear, cutting and drilling applications.

A well-known problem experienced with this type of PCD compact, however, is that the residual presence of solvent/catalyst material in the microstructural interstices has a detrimental effect on the performance of the compact at high temperatures as it is believed that the presence of the solvent/catalyst in the diamond table reduces the thermal stability of the diamond table at these elevated temperatures. For example, the difference in thermal expansion coefficient between the diamond grains and the solvent/catalyst is believed to lead to chipping or cracking in the PCD table of a cutting element during drilling or cutting operations. The chipping or cracking in the PCD table may degrade the mechanical properties of the cutting element or lead to failure of the cutting element. Additionally, at high temperatures, diamond grains may undergo a chemical breakdown or back-conversion with the solvent/catalyst. At extremely high temperatures, portions of diamond grains may transform to carbon monoxide, carbon dioxide, graphite, or combinations thereof, thereby degrading the mechanical properties of the PCD material.

A potential solution to these problems is to remove the catalyst/solvent or binder phase from the PCD material.

Chemical leaching is often used to remove metal-solvent catalysts, such as cobalt, from interstitial regions of a body of PCD material, such as from regions adjacent to the working surfaces of the PCD. Conventional chemical leaching techniques often involve the use of highly concentrated, toxic, and/or corrosive solutions, such as aqua regia and mixtures including hydrofluoric acid (HF), to dissolve and remove metallic-solvent/catalysts from polycrystalline diamond materials. As such mixtures are highly toxic, the use of these carry severe health and safety risks and therefore processes for treating PCD with such mixtures must be carried out by specialised personnel under well-controlled and monitored conditions to minimise the risk of injury to the operators of such processes.

Furthermore, it is typically extremely difficult and time consuming to remove effectively the bulk of a metallic catalyst/solvent from a PCD table, particularly from the thicker PCD tables required by current applications. In general, the current art is focussed on achieving PCD of high diamond density and commensurately PCD that has an extremely fine distribution of metal catalyst/solvent pools. This fine network resists penetration by the leaching agents, such that residual catalyst/solvent often remains behind in the leached compact. Furthermore, achieving appreciable leaching depths can take so long as to be commercially unfeasible or require undesirable interventions such as extreme acid treatment or physical drilling of the PCD tables.

There is therefore a need to overcome or substantially ameliorate the above-mentioned problems through a technique for treating or processing a body of PCD material.

SUMMARY

Viewed from a first aspect there is provided a method of processing a polycrystalline diamond (PCD) material having a non-diamond phase comprising a diamond catalyst/solvent material, the method comprising leaching an amount of the diamond catalyst/solvent from the PCD material by exposing at least a portion of the PCD material to a leaching mixture, the leaching mixture comprising hydrochloric acid and one or more additional mineral acids, the molar concentration of the hydrochloric acid being greater than the molar concentration of the one or more additional mineral acids in the leaching mixture.

The leaching mixture may comprise, for example, one or more mineral acids comprising one or more of sulphuric acid, phosphoric acid, perchloric acid, or nitric acid.

In some embodiments, the molar concentration of the hydrochloric acid is 3M or greater.

In some embodiments, the leaching solution comprises a combination of two or more mineral acids in addition to hydrochloric acid, the molar concentrations of the two or more mineral acids being substantially identical.

The method may further comprise heating the leaching solution to a temperature equal to or greater than the boiling temperature of the leaching mixture during the step of exposing the PCD material to the leaching mixture.

For an average diamond grain size of around 10 microns, the rate of leaching by the acid mixture may, in some embodiments be around 10 microns per hour.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described in more detail, by way of example only, with reference to the accompanying figures in which:

FIG. 1 is a schematic perspective view of a PCD cutter insert for a cutting drill bit for boring into the earth; and

FIG. 2 is a schematic cross section view of the PCD cutter insert of FIG. 1 together with a schematic expanded view showing the microstructure of the PCD material;

The same reference numbers refer to the same respective features in all drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

As used herein, “PCD material” is a material that comprises a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume % of the material. In one embodiment of PCD material, interstices among the diamond gains may be at least partly filled with a binder material comprising a catalyst for diamond and/or a non-diamond phase.

As used herein, “catalyst material for diamond” is a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature at which diamond is thermodynamically more stable than diamond.

The term “molar concentration” as used herein, may refer to a concentration in units of mol/L at a temperature of approximately 25[deg.] C. For example, a solution comprising solute A at a molar concentration of 1 M may comprise 1 mol of solute A per litre of solution.

FIG. 1 shows a PCD cutter insert 10 for a drill bit (not shown) for boring into the earth, comprising a PCD body 20 bonded to a cemented tungsten carbide substrate 30.

FIG. 2 is a cross-section through the PCD cutter insert 10 of FIG. 1. The microstructure 21 of the PCD body 20 is also shown and comprises a skeletal mass of inter-bonded diamond grains 22 defining interstices 24 between the diamond grains, the interstices 24 being at least partly filled with a filler material, namely a solvent/catalyst for diamond comprising, for example, cobalt, nickel or iron.

In accordance with some embodiments of the method, a sintered body of PCD material is created having diamond to diamond bonding and having a second phase comprising solvent/catalyst dispersed through its microstructure together. The body of PCD material may be formed according to standard methods, using HpHT conditions to produce a sintered PCD table. The PCD tables to be leached by embodiments of the method typically, but not exclusively, have a thickness of about 1.5 mm to about 3.0 mm.

It has been found that the removal of non-binder phase from within the PCD table, conventionally referred to as leaching, is desirable in various applications, for example, where it is desired to reattach the polycrystalline diamond disk to a carbide post, which is typically accompanied by re-infiltration of, for example, a binder material in order for such re-attachment to be successful. The carbide grains can potentially block the pathways along which re-infiltration occurs. These blockages prevent the complete re-infiltration of the binder material during the reattachment cycle, which in turn has deleterious consequences for the reattachment process.

Also, the residual presence of solvent/catalyst material in the microstructural interstices is believed to have a detrimental effect on the performance of PCD compacts at high temperatures as it is believed that the presence of the solvent/catalyst in the diamond table reduces the thermal stability of the diamond table at these elevated temperatures.

The reaction rate regarding leaching is considered to be dominated by the chemical rate initially as acid contacts a surface of the PCD table and later by the diffusion rate as the acid diffuses through the pores of the PCD table.

Conventionally, HF—HNO₃ has been shown to be the most effective media for the removal of tungsten carbide (WC) from the sintered PCD table. The problem with HF—HNO₃ is that it is volatile and, when heating this acid, specific technology, for example, gas sealing technology, is required. If such technology is not provided then the application of temperature will reduce the efficacy of HF—HNO₃ due to evaporation of the HF (which is poisonous) and formation of NO species, which are usually gaseous, and thus frequent replenishment of the acid media is required. Furthermore, as outlined above heat would ordinarily be required to accelerate the leaching process in order to render the process commercially feasible. Another problem is that HF—HNO₃ is corrosive to most containment vessels making the reaction difficult to perform.

To improve the performance and heat resistance of a surface of the body of PCD material 20, at least a portion of the metal-solvent catalyst, such as cobalt, may be removed from the interstices 22 of at least a portion of the PCD material 20. Additionally, tungsten and/or tungsten carbide may be removed from at least a portion of the body of PCD material 20.

Chemical leaching is used to remove the metal-solvent catalyst from the body of PCD material 20 either up to a desired depth from an external surface of the body of PCD material or from substantially all of the PCD material 20. Following leaching, the body of PCD material 20 may therefore comprise a first volume that is substantially free of a metal-solvent catalyst. However, small amounts of solvent/catalyst may remain within interstices that are inaccessible to the leaching process. Additionally, following leaching, the body of PCD material 20 may also comprise a volume that contains a metal-solvent catalyst. In some embodiments, this further volume may be remote from one or more exposed surfaces of the body of PCD material 20.

The interstitial material which may include, for example, the metal-solvent/catalyst and one or more additions in the form of carbide additions, may be leached from the interstices 22 in the body of PCD material 20 by exposing the PCD material to a suitable leaching mixture.

According to embodiments, the leaching mixture comprises one or more mineral acids in addition to hydrochloric acid, wherein the molar concentration of the hydrochloric acid present in the leaching mixture is greater than the molar concentration(s) of the other mineral acid(s) in the mixture. The body of PCD material may be exposed to such a leaching mixture in any suitable manner, including, for example, by immersing at least a portion of the body of PCD material 20 in the leaching mixture for a period of time.

According to some embodiments, the body of PCD material may be exposed to the leaching mixture at an elevated temperature, for example to a temperature at which the acid leaching mixture is boiling. Exposing the body of PCD material to an elevated temperature during leaching may increase the depth to which the PCD material may be leached and reduce the leaching time necessary to reach the desired leach depth.

If only a portion of the body of PCD material is to be leached, the body, and if it is still attached to the substrate, the substrate may be at least partially surrounded by a protective layer to prevent the leaching solution from chemically damaging certain portions of the body of PCD material and/or the substrate attached thereto during leaching. Such a configuration may provide selective leaching of the body of PCD material, which may be beneficial. Following leaching, the protective layer or mask may be removed.

Additionally, in some embodiments, at least a portion of the body of PCD material and the leaching solution may be exposed to at least one of an electric current, microwave radiation, and/or ultrasonic energy to increase the rate at which the body of PCD material is leached.

Examples of suitable mineral acids may include, for example, sulphuric acid, phosphoric acid, perchloric acid or nitric acid and/or any combination of the foregoing or other mineral acids.

In some embodiments, hydrochloric acid may be present in the leaching mixture in an amount of around 3 M or greater than around 3 M.

Some embodiments are described in more detail with reference to the following examples which are not intended to be limiting. The following examples provide further detail in connection with the embodiments described above.

Example 1

Cutting elements, each comprising a PCD table attached to a tungsten carbide substrate, were formed by HPHT sintering of diamond particles in the presence of cobalt. The sintered-polycrystalline-diamond tables included cobalt within the interstitial regions between the bonded diamond grains.

The PCD table was leached using a solution comprising hydrochloric acid, nitric acid, phosphoric acid and sulphuric acid with a total molar concentration of the leaching mixture being 4.60 M, the nitric acid, phosphoric acid and sulphuric acid being present in the leaching mixture at equal Molar concentrations and the hydrochloric acid being present in a greater Molar concentration than any of the other mineral acids, for example, around 3M.

The PCD table was leached for 15 hours at a temperature at which the acid leaching mixture was boiling.

At this time the leached depth of the PCD table was determined for various portions of the PCD table, through x-ray analysis. It was found that an average leach depth of 270 microns had been achieved after 15 hours.

Another identical body of PCD material was leached for a total 25 hours using this leaching mixture and the leached depth of the PCD table was again determined for various portions of the PCD table using x-ray analysis. It was found that an average leach depth of 450 microns had been achieved after 25 hours.

Example 2

Cutting elements, each comprising a PCD table attached to a tungsten carbide substrate, were formed by HPHT sintering of diamond particles in the presence of cobalt. The sintered-polycrystalline-diamond tables included cobalt within the interstitial regions between the bonded diamond grains.

The PCD table was leached using a solution comprising hydrochloric acid, nitric acid, phosphoric acid and sulphuric acid with a total molar concentration of the leaching mixture being 4.87 M, the nitric acid, phosphoric acid, perchloric acid and sulphuric acid being present in the leaching mixture at equal Molar concentrations and the hydrochloric acid being present in a greater Molar concentration than any of the other mineral acids, for example, around 3M.

The PCD table was leached for 10 hours at a temperature at which the acid leaching mixture was boiling.

At this time the leached depth of the PCD table was determined for various portions of the PCD table, through x-ray analysis. It was found that an average leach depth of 250 microns had been achieved after 10 hours.

Another identical body of PCD material was leached for a total 33 hours using this leaching mixture and the leached depth of the PCD table was again determined for various portions of the PCD table, through x-ray analysis. It was found that an average leach depth of 600 microns had been achieved after 33 hours.

When compared with the leach depths achievable using conventional leaching solutions, it has been determined that the embodiments including the above leaching mixtures may enable a greater leaching efficiency to be achieved with greater leach depths being achievable in a shorter period of time for example, a leach depth of 585 microns was achieved in 20 hours compared to a conventional acid mixture which is capable of leaching an identical body of PCD in typically 5 days. Furthermore, health and safety handling issues are reduced as the acid leaching mixture is less toxic than other conventional HF-nitric based leaching mixtures.

The preceding description has been provided to enable others skilled the art to best utilize various aspects of the embodiments described by way of example herein. This description is not intended to be exhaustive or to be limited to any precise form disclosed. Many modifications and variations are possible. In particular, whilst some embodiments of the method have been described as being effective in leaching PCD containing VC additives, the method may be equally applicable to the effective leaching of PCD with other additives such as those in the form of other metal carbides including one or more of a carbide of tungsten, titanium, niobium, tantalum, zirconium, molybdenum, or chromium. 

1. A method of processing a polycrystalline diamond (PCD) material having a non-diamond phase comprising a diamond catalyst/solvent material, the method comprising leaching an amount of the diamond catalyst/solvent from the PCD material by exposing at least a portion of the PCD material to a leaching mixture, the leaching mixture comprising hydrochloric acid and one or more additional mineral acids, the molar concentration of the hydrochloric acid being greater than the molar concentration of the one or more additional mineral acids in the leaching mixture.
 2. The method of claim 1, wherein the step of exposing the PCD to a leaching mixture comprises exposing the PCD to the leaching mixture wherein the one or more additional mineral acids comprise one or more of sulphuric acid, phosphoric acid, perchloric acid, or nitric acid.
 3. The method of claim 1, wherein the leaching solution comprises hydrochloric acid at a molar concentration of around 3M or greater.
 4. The method of claim 1, wherein the leaching mixture comprises a combination of two or more mineral acids in addition to hydrochloric acid, the molar concentrations of the two or more additional mineral acids being substantially identical.
 5. The method of claim 1, further comprising heating the leaching mixture to a temperature equal to or greater than the boiling temperature of the leaching mixture during the step of exposing the PCD material to the leaching mixture.
 6. The method of claim 1, wherein the metal-solvent catalyst comprises at least one of: cobalt; nickel; iron.
 7. The method according to claim 1, wherein the PCD table has a thickness of from about 1.5 mm to about 3.0 mm.
 8. The method of claim 1, wherein the step of leaching comprises leaching one or more of a carbide of tungsten, titanium, niobium, tantalum, zirconium, molybdenum, chromium, or vanadium from the PCD material.
 9. The method of claim 1, wherein for an average diamond grain size of around 10 microns, the rate of leaching by the acid leaching mixture is around 10 microns per hour.
 10. (canceled) 